The present invention relates generally to fluxless processes for removing metal oxides from the surface of a substrate. More specifically, the invention relates to a method for the fluxless reflow of solders on a substrate surface, particularly for wafer bumping applications.
Wafer bumping is a process used to make thick metal bumps on the chip bond pads for inner lead bonding. The bumps are commonly made by depositing a solder on the pads and then reflowing (referred to herein as a first reflow) to conduct alloying and to change the shape of the solder bump from a mushroom-shape into a hemispherical-shape. The chip with the first-reflowed bumps is “flipped” to correspond to the footprint of the solder wettable terminals on the substrate and then subjected to a second reflow to form solder joints. These solder joints are referred to herein as inner lead bonds. High-melting point solders (e.g., >300° C.) are normally used in the wafer bumping process because it allows for subsequent assembly steps such as outer lead bonding to proceed using lower-melting point solders (e.g., <230° C.) without disruption of the inner lead bonds.
The shape of the solder bumps after the first reflow is critical. For example, a large bump height is preferable for better bonding and higher fatigue resistance. Further, the bumps formed should preferably be substantially uniform to ensure planarity. Substantially uniform solder bumps having relatively larger bump heights is believed to be associated with an oxide-free bump surface during the first reflow. Currently, there are two major approaches of removing solder oxides during first reflow of the solder bumped wafer. One approach is fluxless soldering using pure hydrogen at a reflow temperature of 400 to 450° C. The major challenge of this approach is the flammable nature of the pure hydrogen, which largely limits the application of this approach. The second approach is applying organic fluxes over the deposited solder bumps, or within a solder paste mixture that has been printed onto the wafer to form the bumps, and reflowing the bumps in an inert environment so that the fluxes can effectively remove initial oxides on the solder surface. However, this approach has its drawbacks. Small voids may form in the solder bumps due to flux decomposition. These voids may not only degrade the electrical and mechanical properties of the formed solder bonds but also destroy the co-planarity of the solder bumped wafer and affect the subsequent chip bonding process. The decomposed flux volatiles can also contaminant the reflow furnace which can increase the maintenance cost. In addition, flux residues are oftentimes left upon the wafer which can cause corrosion and degrade the performance of the assembly.
To remove the flux residues from the reflow processes described above, a post cleaning process may be adopted using chlorofluorcarbons (CFCs) as cleaning agents. However, post-cleaning adds an additional process step and increases the manufacturing processing time. Further, the use of chlorofluorocarbons (CFCs) as cleaning agents is banned due to the potential damage to the earth's protective ozone layer. Although no-clean fluxes have been developed by using a small amount of activators to reduce residues, there is a trade-off between the gain and loss in the amount of flux residues and the activity of the fluxes. Therefore, a catalytic method to assist generating highly reactive H2 radicals, and thus reducing the effective ranges of hydrogen concentration and processing temperature for reducing surface oxides, has been sought by the industry.
Fluxless (dry) soldering has been performed in the prior art using several techniques. One technique is to employ lasers to ablate or heat metal oxides to their vaporization temperatures. Such processes are typically performed under inert or reducing atmospheres to prevent re-oxidation by the released contaminants. However, the melting or boiling points of the oxide and base metal can be similar and it may not be desirable to melt or vaporize the base metal. Therefore, such laser processes are difficult to implement. Lasers are typically expensive and inefficient to operate and require a direct line of sight to the oxide layer. These factors limit the usefulness of laser techniques for most soldering applications.
Surface oxides can be chemically reduced (e.g., to H2O) through exposure to reactive gases (e.g., H2) at elevated temperatures. A mixture containing 5% or greater reducing gas in an inert carrier (e.g., N2) is typically used. The reaction products (e.g., H2O) are then released from the surface by desorption at the elevated temperature and carried away in the gas flow field. Typical process temperatures exceed 350° C. However, this process can be slow and ineffective, even at elevated temperatures.
The speed and effectiveness of the reduction process can be increased using more active reducing species. Such active species can be produced using conventional plasma techniques. Gas plasmas at audio, radio, or microwave frequencies can be used to produce reactive radicals for surface de-oxidation. In such processes, high intensity electromagnetic radiation is used to ionize and dissociate H2, O2, SF6, or other species, including fluorine-containing compounds, into highly reactive radicals. Surface treatment can be performed at temperatures below 300° C. However, in order to obtain optimum conditions for plasma formation, such processes are typically performed under vacuum conditions. Vacuum operations require expensive equipment and must be performed as a slow, batch process rather than a faster, continuous process. Also, plasmas are typically dispersed diffusely within the process chamber and are difficult to direct at a specific substrate area. Therefore, the reactive species cannot be efficiently utilized in the process. Plasmas can also cause damage to process chambers through a sputtering process, and can produce an accumulation of space charge on dielectric surfaces, leading to possible microcircuit damage. Microwaves themselves can also cause microcircuit damage, and substrate temperature may be difficult to control during treatment. Plasmas can also release potentially dangerous ultraviolet light. Such processes also require expensive electrical equipment and consume considerable power, thereby reducing their overall cost effectiveness.
U.S. Pat. No. 5,409,543 discloses a process for producing a reactive hydrogen species (i.e., atomic hydrogen) using a hot filament to thermally dissociate molecular hydrogen in a vacuum condition. The energized hydrogen chemically reduces the substrate surface. The temperature of the hot filament may range from 500° C. to 2200° C. Electrically biased grids are used to deflect or capture excess free electrons emitted from the hot filament. The reactive species or atomic hydrogen is produced from mixtures containing 2% to 100% hydrogen in an inert carrier gas.
U.S. Pat. No. 6,203,637 discloses a process for activating hydrogen using the discharge from a thermions cathode. Electrons emitted from the thermionic cathode create a gas phase discharge, which generates active species. The emission process is performed in a separate or remote chamber containing a heated filament. Ions and activated neutrals flow into the treatment chamber to chemically reduce the oxidized metal surface. However, such hot cathode processes require vacuum conditions for optimum effectiveness and filament life. Vacuum operations require expensive equipment, which must be incorporated into soldering conveyor belt systems, thereby reducing their overall cost effectiveness.
Potier, et al., “Fluxless Soldering Under Activated Atmosphere at Ambient Pressure”, Surface Mount International Conference, 1995, San Jose, Calif., and U.S. Pat. Nos. 6,146,503, 6,089,445, 6,021,940, 6,007,637, 5,941,448, 5,858,312 and 5,722,581 describe processes for producing activated H2 (or other reducing gases, such as CH4 or NH3) using electrical discharge. The reducing gas is generally present at “percent levels” in an inert carrier gas (N2). The discharge is produced using an alternating voltage source of “several kilovolts”. Electrons emitted from electrodes in a remote chamber produce exited or unstable species that are substantially free of electrically charged species, which are then flowed to the substrate. The resulting processes reduce oxides on the base metal to be soldered at temperatures near 150° C. However, such remote discharge chambers require significant equipment costs and are not easily retrofitted to existing soldering conveyor belt systems. In addition, these processes are typically employed for pre-treating the metal surface before soldering rather than removing solder oxides.
U.S. Pat. No. 5,433,820 describes a surface treatment process using electrical discharge or plasma at atmospheric pressure from a high voltage (1 kV to 50 kV) electrode. The electrode is placed in the proximity of the substrate rather than in a remote chamber. The free electrons emitted from the electrodes produce reactive hydrogen radicals—a plasma containing atomic hydrogen—which then pass through openings in a dielectric shield placed over the oxidized substrate. The dielectric shield concentrates the active hydrogen onto those specific surface locations requiring de-oxidation. However, such dielectric shields can accumulate surface charge that may alter the electric field and inhibit precise process control. The described process is only used to flux base metal surfaces.
Accordingly, there is a need in the art to provide an economical and efficient process for fluxless reflow of solder bumped wafer under relatively low temperatures to reduce thermal energy. There is a further need in the art to provide a process and apparatus for fluxless reflow under near ambient or atmospheric pressure conditions to avoid the expense of purchasing and maintaining vacuum equipment. Additionally, there is a need in the art to provide a fluxless reflow process using a non-flammable gas environment.